1. Field of the Invention:
The invention in general relates to sound cancellation apparatus and more particularly to the cancellation of relatively low frequency sounds from large surfaces.
2. Description of the Prior Art:
Any object that vibrates and disturbs its surrounding ambient medium may become an acoustic source by radiating acoustic waves which vary in wavelength (.lambda.) according to their frequency. Very often, the vibration is unwanted and is a source of acoustic noise. Such noise may be radiated for example from reverberating structures, vibrating machinery, large transformers and various other types of apparatus in various ambient mediums.
The most direct means for reducing the sound intensity from a typical acoustic source is to surround the source with an acoustic baffle which cuts off its direct acoustic propagation path. Various absorbing materials exist which have the ability to dissipate sound energy by converting it to heat energy. Such absorbers work well for the high frequency range, however, they are extremely bulky and limited in application for the low frequency range.
Another type of noise cancellation arrangement employs a microphone, amplifier and loudspeaker to measure the noise in a local area relatively distant from the source and to produce equal amplitude and opposite phase acoustic signals to cancel out the sound in the area. Although a significant sound reduction is experienced, it is experienced only for that particular area and not other areas where the sound may be equally objectionable. In addition, such an arrangement is prone to the production of interference patterns which even increase the noise intensity in other locations.
Another type of similar arrangement which achieved limited results placed the microphone very close to an acoustic noise source which approximated a point source. The signal processing circuit for such an arrangement produced a phase opposition signal which was adjustable by suitably adjusting the distance between the microphone and loudspeaker. The limited results obtained with such apparatus, restricted to a point source of acoustic radiation and a single frequency are not applicable to large vibrating surfaces which may be vibrating in a complex mode to produce a wide spectrum of frequencies.
Still another arrangement attempted to use an array of several speakers located near large outdoor transformers with each speaker being electrically tuned from a variable frequency source to reduce single frequency audible signals emitted from the transformers. Although results showed some attenuation for single frequencies over long distances with finite directional angles, the apparatus actually produced intensified sound in other directions. Furthermore the apparatus was very restrictive in regards to operational bandwidth.